The gene
ruinous (rui) belongs to the group of mutants characterized as complex genes with extensive
pleiotropic effects (2). The general habit of the mutant plant is drastically
changed. Rachitic rui plants show extremely reduced stem and petiole
length, the vestigial, deformed leaflets dry up on the tip and the
mutant plants are completely sterile as a result of abortion of the generative
organs.

Mapping of complex, sterile mutants
of this type presents difficulties.
The sterility of homozygous plants makes it a necessity to use
heterozygotes in crossing experiments. A deficiency of mutant segregants, and difficulties with
observation of the phenotype of many morphological markers, is also a
problem. The use of isozymic markers can help to resolve this
puzzle.

The
electrophoresis of isozymes was carried out on horizontal starch gels. The isozymes were visualized by
assays as described by Weeden and Marx (3,4).

Three of the
F2 populations were segregating for a considerable number of known marker loci. Only progenies of F1
plants which showed segregation for rui were used for
mapping studies of this gene. The cross Wt15046 x Wt11238 segregated at loci
i, le, r, tl, wsp, Aat-2,
Aat-3, Acp-1,
Aldo, Est-2, Fum, Gal-2, Gal-3,
Idh, Lap-1, Lap-2, 6pgd-1, Px-1.
The Wt15046 x Wt11143 progeny segregated for many of the above
markers plus Dia-1,
Nag-1, Pgm-2, Prx-3. The Wt15046 x Wt10345 progeny
segregated additionally for
wlo, Acp-5, 6pgd-2.

An
undisturbed Mendelian segregation of phenotypes in F2 progenies
was observed for rui,
wlo and Prx-3. A 3:1 segregation for rui and
wlo was obtained. For
Prx-3 a codominant type of inheritance of 1:2:1 was observed (Table 1). Significant deviation
from random assortment was obtained between rui and
Prx-3 in two F2 progenies (Table 2). None of the
other markers showed linkage with
rui.

These
results indicate that rui is located near Prx-3 on
chromosome 6. The calculated map
distance differed slightly between the two crosses. The close linkage between Prx-3 and
wlo is worth emphasizing, since at the same time there was a lack of linkage
between rui and wlo in the cross Wt15046 x Wt10345 (Table 2). This suggests
that the rui locus is more distal from the centromere than
Prx-3.

More
precise localization of the rui locus has not yet been possible.
There are no more isozyme markers on
chromosome 6 and good morphological markers are difficult to observe on
sterile, strongly changed plants of the rui mutant. Because of this
situation a recent report of the localization of the sbm gene in the Pl
end of chromosome 6 is especially interesting (1 ). The sbm and Arg genes
may allow the location of the rui gene to be determined more precisely.

This research was carried out at the
N.Y. State Agricultural Experiment Station in Geneva, NY (USA). I wish to
express my sincere gratitude to Dr.
N.F. Weeden for allowing this work to be carried out in his laboratory and to Dr. W.K. Swiecicki
for the plant material from the Wiatrowo
genebank.